How does a film camera perform exposure compensation?

Question

I have a Nikon D3x, a Nikon F6 and a Nikon F3/T.

It is my understanding that, on a digital camera, "exposure compensation" actually alters signal amplification, similar but not identical to changing the sensitivity.

Both the F6 and the F3/T offer exposure compensation. Neither of these cameras appear to be altering shutter speed or diaphragm opening in accordance to exposure compensation. Yet the resulting images are properly exposed as per the compensation.

For example, setting the exposure compensation of the F6 to -5 EV does not audibly change its shutter speed, nor does the diaphragm opening visibly change. In fact the duration of the exposure appears to be unaltered.

So how does a film camera perform exposure compensation?

Answer 1

I understand that, on a digital camera, "exposure compensation" actually alters signal amplification, similar but not identical to changing the sensitivity.

In general, this is not how the term is used. Instead, "exposure compensation" means: tell the camera's exposure program in automatic exposure modes to target an exposure brighter or darker than the meter reading. (More at What is exposure compensation?, and at How do DSLRs figure out what aperture to select in P mode?)

One way the program may respond to this is by altering the amplification, increasing the effective ISO. (More at How is ISO implemented in digital cameras?) It can also change the shutter speed or aperture — but that will depend on the mode.

On a film camera, changing the amplification is not an option, so to change the exposure, shutter speed or aperture must change. However, you might not perceive them changing, because the camera doesn't actually stop down until the shutter clicks, and it's hard to hear the difference between, say, ¹⁄60th vs ¹⁄₁₂₅th of a second.

Nikon actually includes the "program chart" for the F6 in the manual:

… so you can see exactly what it does. For example, if the meter returns "EV 5", you'd get an aperture of f/4 and a shutter speed of ¹⁄₆₀th. And, you can see that it's really quite a simple algorithm. As noted in this answer, some cameras let you select between different programs. On some entry level cameras, a setting like "sports mode" actually means "select a program which prioritizes high shutter speeds" — even if there's no direct, named control over the program. Generally, though, if you really care a lot about a particular exposure factor, you would use Aperture or Shutter Priority.

Answer 2

It's not the case that "exposure compensation actually alters signal amplification".

There are only 3 ways to control exposure: aperture, shutter speed and ISO. Exposure compensation is not a magical additional exposure control. It just tells the camera to change the exposure x number of stops by changing the aperture, the shutter speed or (in the case of a digital camera) the ISO.

Actually, "exposure compensation" is a misnomer in my opinion - it should be called "meter compensation". It works (almost) the same way on a film camera as it does on a digital camera. When you point your camera at a scene, and the camera meters the scene at, say, EV 16, if you set exposure compensation to +1, what you are doing is telling the camera to assume the scene is 1 EV less (i.e. EV 15) and to expose accordingly. Depending on your exposure mode (PASM), the +1 exposure is then achieved by the camera altering the aperture or the shutter speed or (in the case of a digital camera) the ISO so as to make an exposure as if the scene were EV 15.

(As noted by Michael in the comments, it's possible that the camera may adjust more than one of the exposure controls in order to achieve the desired exposure compensation, e.g. it may adjust both aperture and shutter speed at the same time.)

I imagine the reason you can't hear or see any difference in shutter speed or aperture in your test case is because everything is happening too fast anyway. It's probably worth explicitly stating that, regardless of aperture setting or exposure compensation setting, the lens diaphragm remains fully open until the moment the shutter release button is pressed.

Answer 3

So how does a film camera perform exposure compensation?

The same way a digital camera performs exposure compensation (EC): it biases the meter calibration in the direction and amount entered so the resulting exposure variables selected by the camera result in less or more exposure than would otherwise be the case.

The reason we sometimes need to do EC is because most cameras can't tell the difference between a black cat in a coal mine and a white cat in an avalanche. Particularly when using a camera with a monochromatic light meter (pretty much all consumer/pro SLR cameras until about 2010 or so), the camera will try to expose every scene as a medium gray. With newer color-based light meters and library-based exposure algorithms, some cameras are getting better at "guessing" the difference in difficult lighting. But the camera still can't read the photographer's mind about what part of the scene we want to be exposed as 'medium brightness'.

Film Cameras

In the case of film cameras either the shutter time (Tv), aperture (Av), or possibly some of both if using Program exposure mode, are altered to accomplish the higher or lower exposure. We can even do EC with a film camera that has no [+/- EC] button or dial. If we are shooting with ISO/ASA 200 film in manual exposure mode and we change the film speed setting to 'ISO/ASA 100' the meter will give a one-stop lower reading for the same amount of light than when it is set at ISO/ASA 200. Any of the camera's automated exposure modes will overexpose the 200 speed film by one stop by changing Tv or Av to return the calculated exposure to the middle. It has the exact same effect as using [+1 EC]. Setting the film speed control to ISO/ASA 800 with 200 speed film will have the same effect as using [-2 EC]. In either case, don't forget to change the film speed setting back after you take your picture!

Both the F6 and the F3/T offer exposure compensation. Neither of these cameras appear to be altering shutter speed or diaphragm opening in accordance to exposure compensation. Yet the resulting images are properly exposed as per the compensation.

For example, setting the exposure compensation of the F6 to -5 EV does not audibly change its shutter speed, nor does the diaphragm opening visibly change. In fact the duration of the exposure appears to be unaltered.

Most of the "shutter" sound you hear with an SLR is the movement of the reflex mirror before and after the exposure. The mirror movement before and after the exposure takes the same amount of time regardless of the shutter time used. Also keep in mind that all shutter times take the same amount of time for each shutter curtain to transit the film plane. The only difference is in the length of delay between the movement of the first curtain to begin opening and the movement of the second curtain to begin closing. At shutter times shorter/faster than your camera's X-sync speed, the second curtain begins to close before the first curtain is fully open. This results in a slit of varying width between two curtains as the second curtain "chases" the first curtain across the film plane

• Let's say your camera takes 50 milliseconds (1/20 second) to raise the mirror. For simplicity's sake, let's assume it takes another 50 ms for the mirror to drop back down. That's 100 ms (1/10 second) of mirror travel for every picture you take with your camera. Your F6 might be that fast, but the F3/T is likely a bit slower than that.
• Let's say your camera has a flash sync speed of 1/60 s (fairly common back in the days of horizontal travel shutters that travelled across the long dimension of the 36x24mm film frame). Let's be generous and give the flash 5 ms (1/200 s) to do it's thing before the second curtain starts to close at 1/60 s (16.7 ms). That means it takes about 11.7 ms (1/85 s) for each shutter curtain to transit from one side of the film plane to the other.
• The total shutter movement time for a 1/2000 s (0.5 ms) exposure with our camera with a flash sync speed of 1/60 s (16.7 ms) is roughly 1/85 s (11.7 ms) + 1/2000 (0.5 ms). Add another 100 ms for mirror movement. Total "shutter sound" time is 112.2 milliseconds.
• The total shutter movement time for a 1/1000 s (1.0 ms) exposure is roughly 1/85 (11.7 ms) + 1/1000 (1.0 ms). Add another 100 ms for mirror movement. Total "shutter sound" time is 112.7 milliseconds
• Can you honestly expect to tell the difference between 112.2 milliseconds, the total "shutter sound" time for a 1/2000 second exposure, and 112.7 milliseconds, the total "shutter sound" time for a 1/1000 second exposure? Or even between 112.2 ms for a 1/2000 s exposure and 116.7 ms, which is the total "shutter sound" time needed for a five stops slower Tv of 1/60 seconds?

If you're looking at the diaphragm opening before the shutter button is pressed to take the exposure, it is being held open at the lens' maximum aperture setting regardless of the aperture value selected. This allows the most light possible to enter the camera for focusing and metering. The aperture is not stopped down until the shutter button is fully pressed. Remember those 50 milliseconds needed to raise the mirror? That's more than enough time for the camera to stop down the aperture. By the time the mirror drops back down after the exposure, the aperture is back to the fully open position as well.

Digital Cameras

In the case of digital cameras either the Tv, Av, ISO, or a combination of them are altered to accomplish a higher or lower exposure when EC is used. Sometimes using EC alters the camera's ISO setting that controls sensor amplification, but often it does not. Whether using EC results in changing the ISO and/or Tv and/or Av all depends on the user's selected exposure mode and settings as well as the camera's program line for a specific scenario and exposure mode.

There are only three things that determine exposure: Tv, Av, and ISO/sensitivity. This is just as true in a digital camera as it is with a film camera.

We can alter 'exposure/brightness' when post-processing a digital image. We can just as easily alter the development time of film in the darkroom to increase or decrease the density of the negative. Neither modification after the fact alters the amount of light that was captured by the film or sensor during the actual exposure event. The only difference is that with digital when we alter 'exposure/brightness' in post-processing we do so non-destructively - the actual raw data collected by the sensor remains unchanged. In the case of film, once we have developed a latent image to create a negative there is no going back and doing it again with a different development time.

It is my understanding that, on a digital camera, "exposure compensation" actually alters signal amplification, similar but not identical to changing the sensitivity.

It seems that some folks think that entering an EC value results in the raw data from the sensor being processed differently than if the same ISO were selected with a different EC value.

This is not the case at all!

The only thing that controls the amount of amplification of the analog information coming off the sensor before it is converted to raw digital data is the ISO setting. An image taken using [-3 EC] that resulted in the camera using ISO 400 will result in the exact same sensor amplification as an image taken using [0 EC] or [+5 EC] that resulted in the camera using ISO 400.

What matters with regard to sensor amplification is what the ISO is set to when the image is taken. Period. How a particular ISO is selected matters not: whether chosen by a manually entered ISO setting or by an automated program resulting from use of EC, a specific ISO value will always result in the same amount of sensor amplification. ISO 100 has the same signal amplification regardless of whether EC is set to -5, 0, or +5 at the time the image is exposed.